Field of the Invention
The present invention relates to the field of magnetic resonance imaging, in particular a multi-slice data acquisition method for a magnetic resonance imaging system.
Description of the Prior Art
Magnetic resonance imaging (MRI) is a technology in which the phenomenon of magnetic resonance is utilized for the purpose of imaging. The basic principles of magnetic resonance are as follows: when an atomic nucleus contains a single proton, as is the case with the nuclei of the hydrogen atoms that are present throughout the human body, this proton exhibits spin motion and resembles a small magnet. The spin axes of these small magnets lack an adhesive pattern, but when an external magnetic field is applied, the small magnets will be rearranged according to the magnetic force lines of the external magnetic field. Specifically, they will align in two directions, either parallel or anti-parallel to the magnetic force lines of the external magnetic field. The direction parallel to the magnetic force lines of the external magnetic field is called the positive longitudinal axis, while the direction anti-parallel to the magnetic force lines of the external magnetic field is called the negative longitudinal axis. The atomic nuclei only have a longitudinal magnetization component, which has both a direction and a magnitude. A radio frequency (RF) pulse of a specific frequency is used to excite the atomic nuclei in the external magnetic field such that their spin axes deviate from the positive longitudinal axis or negative longitudinal axis, giving rise to resonance—this is the phenomenon of magnetic resonance. Once the spin axes of the excited atomic nuclei have deviated from the positive or negative longitudinal axis the atomic nuclei have a transverse magnetization component.
Once emission of the RF pulse has ended, the excited atomic nuclei emit an echo signal, gradually releasing the absorbed energy in the form of electromagnetic waves, such that their phase and energy level both return to the pre-excitation state. An image can be reconstructed by subjecting the echo signal emitted by atomic nuclei to further processing, such as spatial encoding.
The Half-Fourier Acquisition Single-shot Turbo spin Echo (HASTE) sequence is a turbo spin echo (TSE) technique that uses a single-shot method to acquire sufficient data for an entire image in the repetition time (TR) of a single excitation pulse. Multi-slice HASTE is widely used in clinical imaging research to obtain transverse relaxation time (T2)-weighted images. Due to its relatively short acquisition time, multi-slice HASTE is often used in breath-hold abdominal imaging. A non-ideal slice shape of a selective RF pulse will lead to excitation of neighboring regions, which must be avoided. Adjacent slices will be partially saturated so that the signal generated will be reduced. Unless an extremely long repetition time (TR) is used, crosstalk between slices may cause artefacts or a decrease in signal-to-noise ratio in two-dimensional acquisition. When a TSE sequence is used, these phenomena are more pronounced. Furthermore, an RF pulse sequence for a given slice can serve as an off-resonance magnetization transfer (MT) pulse for another slice, leading to contrast alteration and signal attenuation induced by magnetization transfer. Specifically, compared to TSE sequences having shorter echo chains, the HASTE sequence will induce a greater magnetization transfer effect.